Researchers have reported a perceptual asynchrony when vision binds two separate sources of information to form a unitary perception. However, the determinants of binding remain unsettled. Here, we investigated whether spatial attention modulates the perceptual asynchrony between two sources of information to be bound (shape and color). If binding was fixed in early vision, the temporal asynchrony to bind color and shape would itself be fixed (Moutoussis and Zeki, 1997). In contrast, temporal asynchrony between the information sources should be found if spatial attention modulates binding.

We report a new method that directly manipulates the physical asynchrony between two information sources (source 1: alternation of red/green; source 2: alternation of square/circle) to determine when they are in perceptual synchrony vs. asynchrony. When the sources are perceptually synchronous, the observer perceives the alternation of the red square/green circle stimuli. When the sources are perceptually asynchronous, a new perception emerges (red circle vs. green square).

In each trial of a 2AFC discrimination task, a fixation cross appeared for 1 s, followed by one row of 6 linearly positioned colored shapes (e.g., all red squares) equally distributed left and right of the fixation cross. We manipulated attention with a non-predictive white cue presented for 67 ms above the middle left or middle right positions. All six stimuli would then alternate to, e.g., green circle, but in one randomly chosen position, information would be asynchronous (as described above). We tested 7 physical lags from 13 to 93ms between the two sources, to derive the critical lag required for the 75% correct discrimination of the emergent stimulus (e.g., red circle vs. green square).

Contrary to a fixed binding hypothesis, we found that attention to left and right positions modulated the perceptual asynchrony required to bind identical colors and shapes in discriminable perceptions.